http://www.aroundgreece.com/ancient-greece-history/prehistory-ancient-greece.php
http://everything2.com/title/Prehistoric+Greece
http://everything2.com/title/Prehistoric+Greece
http://en.wikipedia.org/wiki/Hellenistic_Greece
http://www.ellopos.net/elpenor/greek-texts/ancient-greece/history-of-ancient-greek-art-2.asp
The periods you mentioned β Classical, Archaic, and Hellenistic β are indeed part of ancient Greek art history, with the Classical period being the most famous. During this time, many sculptures were created, including well-known works such as the sculpture of Zeus at Olympia or the Venus de Milo.
While it is true that most sculptures from this period do not have attributed artists, you can still identify the cultural context by indicating that the sculpture is from the Classical period. Greek sculptures from this time are often associated with the overall artistic and cultural accomplishments of the ancient Greeks.
If you are specifically looking for a sculpture from the Rome era, you can choose from Roman sculptures that were influenced by Greek art. Roman sculptures often replicated Greek sculptures or were created in a similar style, showcasing the impact and admiration for ancient Greek art.
To summarize, when choosing a sculpture for your assignment and you are uncertain of the artist or specific prehistoric culture, you can write that the sculpture is from the Classical period, which falls within the broader ancient Greek art context.